1. Field of the Invention
This invention relates to an electromagnetically driven camera shutter, particularly focal plane shutters, and involves an auxiliary spring to compensate for insufficiencies in the drive force exerted by the electromagnetic portions of the shutter drive device.
2. Description of the Prior Art
Increased use of electronic controls in cameras has resulted in various proposals for electromagnetic drive devices to drive shutter blades across a shutter's optical path. Because such electromagnetic drive devices must be small enough to be incorporated into compact cameras, it is difficult to utilize mechanisms sufficiently powerful to perform the desired functions. The energy available for consumption also limits this driving force. Thus, it is often necessary to try to utilize lighter shutter blades, avoid friction between drive members, etc. so as to drive mechanisms such as focal plane shutters. For compact electromagnetic drive sources utilizing internal magnetic type drive devices, it is necessary that the permanent magnet be compact and powerful. Anisotropic rare earth magnets have been proposed for this purpose. However, the outputs of such magnets are still insufficient for compact drives. This is especially so because it is difficult to distribute the flux evenly with rare earth magnets. This makes it impossible to furnish an electromagnetic drive force which is even over the entire displacement phase of the shutter blades. Hence, it is impossible to provide a large stroke to drive the shutter blades.
If the stroke is not sufficiently large, not only is it difficult to obtain a strong enough drive force, but the accuracy and reliability of the shutter blade control is decreased.